In order to recover light mono-olefins, ethylene, propylene and butenes from a process gas stream containing other hydrocarbon constituents such as a cracked gas, the prior art has typically employed multi-stage rectification and cryogenic chilling trains. Especially successful processes have included McCue, Jr. et al. U.S. Pat. No. 4,900,347 and McCue, Jr. U.S. Pat. No. 5,035,732. In general these processes fall into two groups, one known as a "front-end" process, the other as a "tail-" or "back-end" process. In the front end process, a full-range stream containing both light and heavy components ranging from hydrogen up to C5's and heavier is processed over a fixed bed of selective hydrogenation catalyst. The catalyst is operated to effect complete removal of simple acetylene and removal of a majority of the methylacetylene and propadiene if present. A majority of the butadiene and dienes are also hydrogenated. The recovery of butadiene and heavier dienes upstream of the reactor system is important because of their commercial value and their tendency to foul the hydrogenation process when the heavy materials remain in the process stream.
In the tail-end process, the full range stream is first fractionated, followed by removal of acetylenes from the individual concentrated streams by reacting these alkynes with hydrogen over selective hydrogenation catalysts. Such a process increases capital cost and is energy-intensive.
In the past numerous methods have been employed to overcome the inefficiencies of these processes. For example, a distillation system can be used upstream of a front-end hydrogenation reactor. However, the distillation tower must function as a deethanizer or partial depropanizer in order to place enough C.sub.3 material in the bottoms to lower the boiling point of the bottoms material below the temperature where C.sub.4 and heavier dienes will polymerize and foul the tower reboiler. This requires the tower to be large in size, expensive due to its high pressure and low temperature construction and energy intensive.
Another method that has been employed is to sufficiently cool the stream at high pressure prior to the reactor to condense most of the butadiene and heavier hydrocarbons. The liquid is then distilled at low pressure (to avoid polymerization and fouling) to produce liquid containing C.sub.3 and heavier material. However, the tower overhead vapors, which contain large quantities of light hydrocarbon material, must be recycled into the cracked gas compressor.
Still another method used in the prior art is to employ a back-end acetylene hydrogenation system with a deethanizer. This method removes butadiene before the hydrogenation reactor and with appropriate recycles and deethanizer operating conditions avoids C.sub.4 and heavier diene polymerization. However, such a process requires large amounts of energy for hydrogen recovery and purification and deethanizing. Back end reactor systems typically require frequent regenerations to maintain selectivity and minimize the potential for runaway reactions.
By contrast, a front-end hydrogenation reactor provides cooler operating conditions because the gases are greatly diluted by the presence of hydrogen and methane. The front-end reactor also enables the hydrogen in the process stream to be used for hydrogenation, minimizes catalyst fouling so that frequent on-site catalyst regeneration is not required, minimizes green oil production, and provides ethylene and propylene gain across the reactor so that production from the plant is increased.
The use of a front-end reactor and a depropanizer or deethanizer as the front end column has been found to provide greater stability and flexibility for the operation of an ethylene plant, see McCue, Jr. U.S. Pat. No. 5,414,170, so that it may be employed over a range of feedstocks from ethane and propane to atmospheric gas oil and the system is less subject to turndown or composition changes resulting from the cyclical operation of the pyrolysis furnaces.
Accordingly, it would represent a notable advancement in the state of the art if a system were provided which economically removes butadiene and heavier dienes from a cracked gas process stream prior to hydrogenation in an olefin production plant. To this end, the present inventor has developed a butadiene and heavier diene removal system for ethylene plants with front end hydrogenation systems. The butadiene recoverable with this removal process has, in many applications, an economic value greater than the mono-olefins and may be processed into a variety of petrochemicals such as but not limited to butyl rubber, hexamethylene diamine, adipic acid, 1,4-butanediol, sulfolane and chloroprene.